1. Field of the Invention
The present invention relates to an ink jet printing apparatus and an ink jet printing method that complete an image in a predetermined print area on a print medium by performing a bidirectional printing scan of a print head capable of ejecting ink.
2. Description of the Related Art
Ink jet printing apparatus have been used widely as printing apparatus with functions of printer, copying machine and facsimile and as output devices for composite electronic devices such as computers and word processors and for workstations. These printing apparatus form an image (including letters) on a print medium such as paper and plastic sheets according to image information (letter information included). Since ink jet printing apparatus eject ink from a print head onto a print medium, a resolution of the printed image can be enhanced and a printing speed increased with greater ease than other types of printing apparatus. They also have an advantage of quiet operation and low cost. There are growing needs for color image printing and many ink jet printing apparatus that meet this demand have been developed.
For further improvement of the printing speed, such an ink jet printing apparatus uses a plurality of print heads each with an array of printing elements (also referred to as a multihead). Each of the printing elements includes an ink ejection opening and a corresponding ejection energy generation element. As the ejection energy generation element, a heater (heating resistive element) or a piezoelectric element may be used. In the following description, the ink ejection opening and the ejection energy generation element combine to form a “nozzle”. In an ink jet printing apparatus that prints a color image, in general, a plurality of print heads each having such printing elements integrally arrayed are provided.
Japanese Patent Laid-Open No. 60-107975 discloses an ink jet printing apparatus of a so-called serial scan type as an example of the ink jet printing apparatus that uses a print head having a plurality of ink ejection openings formed in lines. The serial scan type ink jet printing apparatus forms an image on a print medium by ejecting ink from the nozzles of the print head as it moves the print head in a main scan direction and by alternating this main scan operation with a feeding operation that feeds the print medium in a sub-scan direction.
Japanese Patent Laid-Open No. 60-107975 also discloses a multi-path printing method that forms a high quality image without density variations by taking into account small variations in ink ejection characteristics among nozzles that occur in a print head manufacturing process and which affect an ink ejection volume and direction. The multi-path printing method completes an image in a predetermined print area with a plurality of print head scans and can print single lines in the main scan direction by using a plurality of nozzles. Using a plurality of nozzles in printing a predetermined unit print area, as described above, can minimize the effects of ejection characteristic variations among nozzles and form a high-quality image with no density variations. More specifically, Japanese Patent Laid-Open No. 60-107975 describes a 2-pass printing method that completes the printing operation on a 4-pixel-high print area with two scans. In the 2-pass printing, a first scan prints the 4-pixel-high band area in a hounds-tooth check pattern by using a thinning pattern; and a second scan prints the same band area in a reverse check pattern by using a reverse thinning pattern.
Japanese Patent Laid-Open No. 06-336015 also describes a construction that combines the multi-pass printing method with a bidirectional printing method that ejects ink as the print head moves both in a forward and a backward direction. More specifically, in a 3-pass bidirectional printing method, print ratios during a first, second and third scan are set at 25%, 50% and 25%.
For the ink jet printing apparatus to print a high-quality image at high speed, small ink droplets need to be ejected from a print head at high frequency. In that case, however, there is a possibility of a stripe of print variations being formed in printed images I(n) and I(n+1), as shown in FIG. 13.
FIG. 13 is an explanatory diagram showing an operation of a single-pass printing that completes printing on a predetermined print area with one scan of the print head H. An image printed based on print data D(n) during the n-th scan of the print head H is shown at I(n), which has blank lines formed at the upper and lower parts thereof where no ink droplets land. An image printed based on print data D(n+1) during the (n+1)st scan of the print head H is shown at I (n+1), which also has blank lines formed at the upper and lower parts thereof. Such lines of image defects often occur in areas with high ink dot densities (high print duties).
FIG. 14 is a diagram explaining a possible cause for such lines of image defects that occur during image printing, with ink droplets shown being ejected from the print head H toward a print medium P. This diagram represents a solid printing, a printing operation with a dot density (print duty) of 100% in which all nozzles (e.g., 256 nozzles) in the print head H are activated to eject ink. In this printing state, ink droplets ejected from those nozzles situated near the ends of the nozzle array (nozzles near the upper and lower ends in FIG. 14) are deflected toward the center of the nozzle array as they fly toward the print medium P. The reason for this phenomenon is as follows. Since all the nozzles are activated at high frequency to eject ink, air immediately surrounding the ejected ink droplets also move in the same direction as the ink droplets. That is, as the air immediately surrounding the droplets moves, the surrounding air is negatively pressurized, causing outside air of the surrounding air to move toward the decompressed space, thus generating an air flow directed toward the center of the nozzle array, as indicated by an arrow in FIG. 14. This air flow causes the ink droplets ejected from the nozzles near the ends of the nozzle array to deflect inwardly to the center of the nozzle array (this phenomenon is also referred to as an “end nozzle droplet deflection”). As a result of this end nozzle droplet deflection, ink droplets ejected from those nozzles situated near the ends of the nozzle array land at positions, i.e., dot forming positions, deviated from intended ones, leading to a possibility of lines of image defects being formed as shown in FIG. 13.
To avoid such an end nozzle droplet deflection phenomenon, a method may be conceived that increases the volume of ink droplets to make them unlikely to be easily affected by the air flow. Making the ink droplets large, however, contributes to showing more distinctively the granularity of the dots formed on a print medium, degrading the quality of printed image. Further, lowering the ink ejection frequency or reducing the number of nozzles provided in the print head or lowering the nozzle arrangement density to minimize the end nozzle droplet deflection phenomenon can lead to a reduction in the printing speed.
The end nozzle droplet deflection phenomenon depends on the density (print duty) of dots formed in one scan of the print head. So, the similar phenomenon can occur when the density (print duty) of dots is high not only during the single-pass printing operation, such as shown in FIG. 13, but also during a multi-pass printing operation, such as described in Japanese Patent Laid-Open No. 60-107975.
FIGS. 15A to 15C show a relation between the scan directions of the print head in the bidirectional printing method and dots formed of ink droplets that have landed on a print medium.
In a bidirectional printing operation, the print head H ejects ink from its nozzles N as it moves in both the forward direction of arrow X1 and the backward direction of arrow X2 in FIG. 15A. FIG. 15B shows landing positions of ink droplets when the print head H scans in the forward direction. FIG. 15C illustrates landing positions of ink droplets when the print head H scans in the backward direction. D1 represents a main ink droplet ejected from the nozzles N and D2 represents a sub ink droplet ejected from the nozzles N following the main droplet D1. The print head in FIG. 15A has the direction of ink ejection from the nozzles N slightly inclined toward the forward direction (arrow X1). So, during the forward scan the sub droplet D2 lands at the same position as the main droplet D1, as shown in FIG. 15B. During the backward scan, however, the sub droplet D2 lands at a position deviated from the main droplet D1 in the backward direction (direction of arrow X2), as shown in FIG. 15C.
In a multi-pass printing operation that completes an image in a predetermined print area with an odd number of scans, a first region and a second region, described below, are alternated in position on a print medium. The first region is an area in which an image printing is completed by an even number of forward scans and an odd number, which is one less than the even number, of backward scans, i.e., the area that is completed with a higher ratio of dots printed by forward scans than that of backward scans and in which the printing starts and ends with a forward scan. The second region is an area in which an image printing is completed by an even number of backward scans and an odd number, which is one less than the even number, of forward scans, i.e., the area that is completed with a higher ratio of dots printed by backward scans than that of forward scans and in which the printing starts and ends with a backward scan. Since the first and the second region are alternated in position, density variations can occur in the printed image.
A main cause for the density variations is the fact that the second region with a higher ratio of dots printed by backward scans as shown in FIG. 15C has a wider ink landing area in unit area than that of the first region with a higher ratio of dots printed by forward scans as shown in FIG. 15B. A difference in ink landing area between the two regions can lead to density variations.
In a multi-pass printing that completes the printing operation in a predetermined print area with an odd number of scans, when an image is printed using a plurality of colors of ink droplets, the alternate formation of the first and the second region can result in color variations in a printed image.
Suppose, for example, a blue color is expressed by overlapping a magenta (M) ink and a cyan (C) ink using a print head with nozzle arrays formed to eject yellow (Y), magenta (M), cyan (C) and black (K) inks. In this case, in forward scans the cyan (C) ink lands first, followed by the magenta ink. In backward scans the magenta (M) ink lands first, followed by the cyan (C) ink. This difference in the ink droplet landing order can produce a color difference to a degree that is visible on a printed image. The reason for this is that the ink that has landed first tends to be dominant in color over a subsequently landing ink. The alternate formation of the first and the second region can cause color differences between these regions, which in turn result in color variations.
As described above, Japanese Patent Laid-Open No. 06-336015 describes a 3-pass bidirectional printing method in which 1st, 2nd and 3rd scan is set at print ratios of 25%, 50% and 25% respectively. In this configuration, when we look at a unit print area printed by a total of three scans, it is possible to make a print ratio for all forward scans and a print ratio for all backward scans equal at 50%. This can reduce density variations. It should be noted, however, that since the second scan has a high print ratio of 50%, the density (print duty) of dots formed by the second scan becomes high, which in turn may cause the end nozzle droplet deflection phenomenon of FIG. 14, resulting in lines of image defects.
The density (print duty) of dots formed in one scan can be reduced by lowering the ink ejection frequency or increasing the number of scans required to complete the image printing in a predetermined print area, in order to suppress the end nozzle droplet deflection phenomenon. This, however, can cause a reduction in the printing speed.